On Wed, 19 Jan 2022, anthony wrote: > You all know the story of how the cobbler's children are the worst shod, > I expect :-) Well, the superblock to my raid (containing /home, etc) has > disappeared, and I don't have a backup ... (well I do but it's now well > out of date). > > So, a new hard drive is on order, for backup ... > > Firstly, given that superblocks seem to disappear every now and then, > does anybody have any ideas for something that might help us track it > down? The 1.2 superblock is 4K into the device I believe? So if I copy > the first 8K ( dd if=/dev/sda4 of=sda4.img bs=4K count=2 ) of each > partition, that might help provide any clues as to what's happened to > it? What am I looking for? What is the superblock supposed to look like? Yes, 4K offset. Yes, that dd command will get what you want it to. It hardly matters what the superblock should looks like, because it won't be there. The thing you want to know is: what is there? i.e. you see random bytes and need to guess what they mean, so you can guess where they came from. Best to post the "od -x" output and crowd-source. Are you sure the partition starts haven't changed? Was the array made of whole-devices or of partitions? If you want to find out if the superblock got moved, the maybe searching for the magic number is best. Look a the start of super1.c in mdadm. The first 4 bytes of the superblock are 0xa92b4efc little-endian. So: FC 4E 2B A9 The next 4 bytes as 01 00 00 00 ( the major version) Then the feature map - possibly 0. Then 4 zero bytes. If you see something that looks like that, it worth trying to point mdadm at it. Create a loop device over the it with an appropriate offset, and ask mdadm --example to look at it. > > Secondly, once I've backed up my partitions, I obviously need to do > --create --assume-clean ... The only snag is, the array has been > rebuilt, so I doubt my data offset is the default. The history of the > array is simple. It's pretty new, so it will have been created with the > latest mdadm, and was originally a mirror of sda4 and sdb4. > > A new drive was added and the array upgraded to raid-5, and I BELIEVE > the order is sdc4, sda4, sdb1 - sdb1 being the new drive that was added. > > Am I safe to assume that sdc4 and sda4 will have the same data offset? > What is it likely to be? And seeing as it was the last added am I safe > to assume that sdb1 is the last drive, so all I have to do is see which > way round the other two should be? I would suggest creating some sparse files the same size as the device, create loop devices over them, and creating the array in the sequence you remember doing it - using "--assume-clean" to avoid rebuilds that would make those sparse files less sparse. Then look at the metadata written and assume it is will similar to that which was written to your array. NeilBrown > > At least the silver lining behind this, is that having been forced to > recover my own array, I'll understand it much better helping other > people recover theirs! > > Cheers, > Wol > >
